Spectroscopic techniques are extremely sensitive. Single atoms and even different isotopes of the same atom can be detected among 1020 or more atoms of a different species. (Isotopes are all atoms of an element that have unequal mass but the same atomic number. Isotopes of the same element are virtually identical chemically.) Trace amounts of pollutants or contaminants are often detected most effectively by spectroscopic techniques. Certain types of microwave, optical, and gamma-ray spectroscopy are capable of measuring infinitesimal frequency shifts in narrow spectroscopic lines. Frequency shifts as small as one part in 1015 of the frequency being measured can be observed with ultrahigh resolution laser techniques. Because of this sensitivity, the most accurate physical measurements have been frequency measurements.

Spectroscopy now covers a sizable fraction of the electromagnetic spectrum. The table summarizes the electromagnetic spectrum over a frequency range of 16 orders of magnitude. Spectroscopic techniques are not confined to electromagnetic radiation, however. Because the energy E of a photon (a quantum of light) is related to its frequency ν by the relation E = hν, where h is Planck’s constant, spectroscopy is actually the measure of the interaction of photons with matter as a function of the photon energy. In instances where the probe particle is not a photon, spectroscopy refers to the measurement of how the particle interacts with the test particle or material as a function of the energy of the probe particle.

Electromagnetic phenomena

approximate wavelength range (metres)

approximate frequency range (hertz)

radio waves

10–1,000

3 × 105–3 × 107

television waves

1–10

3 × 107–3 × 108

microwaves, radar

1 × 10−3–1

3 × 108–3 × 1011

infrared

8 × 10−7–1 × 10−3

3 × 1011–4 × 1014

visible light

4 × 10−7–7 × 10−7

4 × 1014–7 × 1014

ultraviolet

1 × 10−8–4 × 10−7

7 × 1014–3 × 1016

X-rays

5 × 10−12–1 × 10−8

3 × 1016–6 × 1019

gamma rays (γ rays)

<5 × 10−12

>6 × 1019

An example of particle spectroscopy is a surface analysis technique known as electron energy loss spectroscopy (EELS) that measures the energy lost when low-energy electrons (typically 5–10 electron volts) collide with a surface. Occasionally, the colliding electron loses energy by exciting the surface; by measuring the electron’s energy loss, vibrational excitations associated with the surface can be measured. On the other end of the energy spectrum, if an electron collides with another particle at exceedingly high energies, a wealth of subatomic particles is produced. Most of what is known in particle physics (the study of subatomic particles) has been gained by analyzing the total particle production or the production of certain particles as a function of the incident energies of electrons and protons.

The following sections focus on the methods of electromagnetic spectroscopy, particularly optical spectroscopy. Although most of the other forms of spectroscopy are not covered in detail, they have the same common heritage as optical spectroscopy. Thus, many of the basic principles used in other spectroscopies share many of the general features of optical spectroscopy.

The distance between successive crests in a wave is called its wavelength. The various forms of electromagnetic radiation differ in wavelength. For example, the visible portion of the electromagnetic spectrum lies between 4 × 10−7 and 8 × 10−7 metre (1.6 × 10−5 and 3.1 × 10−5 inch): red light has a longer wavelength than green light, which in turn has a longer wavelength than blue light. Radio waves can have wavelengths longer than 1,000 metres, while those of high-energy gamma rays can be shorter than 10−16 metre, which is one-millionth of the diameter of an atom. Visible light and X rays are often described in units of angstroms or in nanometres. One angstrom (abbreviated by the symbol Å) is 10−10 metre, which is also the typical diameter of an atom. One nanometre (nm) is 10−9 metre. The micrometre (μm), which equals 10−6 metre, is often used to describe infrared radiation.

The decomposition of electromagnetic radiation into its component wavelengths is fundamental to spectroscopy. Evolving from the first crude prism spectrographs that separated sunlight into its constituent colours, modern spectrometers have provided ever-increasing wavelength resolution. Large-grating spectrometers (see belowPractical considerations: Methods of dispersing spectra) are capable of resolving wavelengths as close as 10−3 nanometre, while modern laser techniques can resolve optical wavelengths separated by less than 10−10 nanometre.

The frequency with which the electromagnetic wave oscillates is also used to characterize the radiation. The product of the frequency (ν) and the wavelength (λ) is equal to the speed of light (c); i.e., νλ = c. The frequency is often expressed as the number of oscillations per second, and the unit of frequency is hertz (Hz), where one hertz is one cycle per second. Since the electromagnetic spectrum spans many orders of magnitude, frequency units are usually accompanied by a Latin prefix to set the scale of the frequency range. (Seemeasurement system: The metric system of measurement: The International System of Units for a table of the prefixes commonly used to denote these scales.)